The direct reduction of iron oxide, in forms such as pellets or lump ore, to metallic iron in the solid state has become a commercial reality throughout the world in recent years. The combined annual capacity of direct reduction plants currently in operation or under construction is in excess of 15 million metric tons of direct reduced iron product, which is used primarily for feedstock in electric arc steelmaking furnaces. The world demand for additional direct reduced iron is projected to increase at a substantial rate for many years to satisfy a growing world need for such feedstock, as additional electric arc furnace steelmaking plants are constructed.
The majority of the plants producing direct reduced iron utilize natural gas as the source of reductant. The natural gas is reformed to produce the reductants CO and H.sub.2. A few plants utilize coal as the source of reductant in rotary kiln processes, such as the SL/RN process, which directly react coal in situ in the kiln without separately gasifying the coal to CO and H.sub.2. The rotary kiln processes have an inherent coal utilization inefficiency in that approximately two-thirds of the coal is burned in the kiln to supply heat and only one-third is used to supply the reducing gas for direct reduction. This inefficiency results in a coal requirement of 5.0 to 6.0 Gcals (Gigacalories) per metric ton of direct reduced iron produced. This is in contrast to 3.0 to 3.5 Gcals of natural gas required per metric ton of direct reduced iron produced in the more efficient natural gas processes, such as the Midrex, Purofer or Armco process.
There are many processes, not yet commercialized, which gasify coal by partial oxidation with oxygen and steam to produce a gas which is then utilized in different manners in the direct reduction of iron. The principal reasons none of these processes has been commercialized are either that such processes are too complex or impractical for commercialization, or the coal requirements are too high. The basic problem which causes an impractical process or a high total coal requirement is that the hot gas issuing from the coal gasifier is too low in reductants (CO plus H.sub.2) relative to oxidants (CO.sub.2 plus H.sub.2 O vapor) to be directly utilized efficiently in the direct reduction of iron.
In the present invention, the hot gas produced by the coal gasifier is upgraded in reductants relative to oxidants within the reduction furnace by reaction with carbon and is also desulfurized by reaction with lime to produce a gas which can be utilized efficiently in the direct reduction of iron, since the upgrading, desulfurizing and direct reduction processes are carried out in the same shaft. The spent reducing gas from the reduction furnace is cooled and scrubbed of dust, then becomes a source of clean, low-sulfur fuel gas to be utilized elsewhere. This combination of direct reduction of iron with fuel gas production has particular utility in an integrated steel plant which currently employs natural gas as fuel gas to supplement coke oven gas for reheating and heat treating operations. The direct reduced iron may be used as feed for basic oxygen steelmaking, or as part of the burden in a blast furnace to increase its hot metal output, or as feed for an electric arc furnace. The fuel gas produced can replace all or part of the natural gas currently used as fuel in the steel plant.
The present invention requires approximately 6.1 Gcals of coal to be gasified plus 0.6 Gcal of carbon for reaction in the furnace plus 0.8 Gcal of coal to produce electricity for gasification oxygen, to produce one metric ton of direct reduced iron while producing 3.6 Gcals of clean fuel gas. The Gcals consumed in producing one metric ton of direct reduced iron are therefore approximately 3.9, as shown in Table IV. It should be noted that only 80% of the carbon added to the furnace charge is reacted and the excess carbon is discharged with the direct reduced iron. This excess carbon can be magnetically separated from the direct reduced iron and recycled to further reduce the energy consumption of the process.